1. Field of the Invention
The present invention relates to a method of detecting defects or imperfections of patterns formed on an object substrate or substrates such as lithography masks used in the microfabrication of various microelectronic devices. The invention also relates to a device suitable for practicing such a detecting method.
2. Discussion of the Prior Art
Inspection to detect defects or imperfections of patterns formed on a substrate is required in various fields of technology. Typically, such inspection is carried out to detect defects of lithography masks, and reticles used to produce such photomasks.
A method of detecting defects of reticles is disclosed in "Submicron lithography (General Technical Treatise), page 425, published by Science Forum, in 1985. According to the disclosed method, design data of a reticle is processed into reference data which represents a pattern that consists of an aggregate of minute picture elements. Inspection data obtained as a result of inspection of a pattern of an actually produced reticle is compared with the reference data, whereby the pattern of the produced reticle is inspected for any defects. In this method, however, a computer used for the data comparison requires a memory having a large capacity, since all of the reference data must be stored in the memory. In the case where the inspection data representative of the inspected reticle pattern is temporarily stored in the memory, the required memory capacity is doubled. In particular, where the object to be inspected is masks used in x-ray photolithography, which is increasingly practiced in the industry, the lines forming a pattern are often less than one micron in thickness, and the size of the picture elements tends to be accordingly small. This indicates an enormous amount of data representative of the pattern. For example, the required memory capacity amounts to 62.5 giga bytes, where an area of 50.times.50 mm.sup.2 is divided into picture elements of 0.2.times.0.2 .mu.m.sup.2.
The inspection of objects other than x-ray photolithography masks does not involve such an enormous amount of data. At any rate, it is desirable to minimize the required memory capacity, in order to reduce the cost of equipment for inspecting the objects.
The above-identified "Submicron lithography (General Technical Treatise)" also discloses at pages 417-419 an inspecting method in which two patterns are concurrently scanned by respective two laser beams or fluxes. The thus obtained two sets of data are compared with each other, and corresponding portions of the two patterns are determined to be defective if the data representative of one of the two corresponding portions critically differs from that of the other portion. This method eliminates a large-capacity memory. In this respect, the method contributes to lower the cost of the inspecting equipment. However, the use of two laser beams requires two separate optical systems, and makes the inspecting equipment cmplicated. While it is also possible to concurrently scan an increased number of patterns for increasing the inspecting efficiency, this approach needs an accordingly increased number of optical systems. Therefore, the number of the laser beams within an actually justifiable range is necessarily limited.
While the inspecting methods for detecting defects of patterns of photolithography masks and recticles have been discussed, there exist similar problems in the field of inspecting patterns of other kinds of objects such as printed-circuited boards.